Resistance Thermometers > 03 - Measurement principle

Resistance thermometers are normally connected to a Wheatstone Bridge type measuring circuit, and the circuit is balanced when the relationship R4.R2 = R3.R1 is respected and therefore no current flows through the null detector, because if this relationship is true, the potentials at points a and b are identical. To use this circuit as a resistance thermometer measuring instrument, we will have the following configurations:

2-wire connection

As can be seen in the figure, two relatively low resistance conductors RL1 and RL2 are used to connect the Pt-100 sensor (R4) to the bridge of the measuring instrument.

In this arrangement, the resistance R4 comprises the resistance of the Pt-100 plus the resistance of the conductors RL1 and RL2. This means that the wires RL1 and RL2, unless they are of very low resistance, can increase the resistance of the sensor appreciably. See below the table of maximum cable lengths for this type of connection. 

3-wire connection

In this situation, the third wire ends up compensating the line resistance, regardless of the variation in ambient temperature along the connecting wires. This type of connection guarantees relative accuracy even with large distances between the sensor element and the measuring circuit.

4-wire connection

This assembly is the most accurate available for RTDs. With two connections at each terminal of the bulb, there is a complete balance of the wire resistances. It is not widely used industrially, only in cases where reading accuracy is required. It is most frequently used in the standards required in calibration laboratories.